The research and development of silicon photonics, or integrating photonics and electronics onto silicon chips, has gained significant advancement over the past decade. Silicon photonics has now found high-impact technological applications in optical communications and optical interconnections for data centers. Silicon photonics features the key merits of (i) compatibility with mature complementary-metal-oxide-semiconductor (CMOS) fabrication processes, (ii) optical transparency in the 1300-1600 nm-range telecommunication wavelengths, which are technologically important, (iii) high refractive index contrast between silicon and silica that enables photonic devices with compact footprint and thus favors potentially large-scale integration, (iv) low waveguide propagation loss (1.7 dB/cm), and (v) high data transmission rate (few tens of Gb/s).
Various research groups from academia and information technology and computer industries worldwide have demonstrated silicon photonic devices including wavelength-selective optical filters, multiplexers/demultiplexers (MUXs/DEMUXs), optical delay lines, electro-optical modulators, optical switches and routers, photodetectors, hybrid-integrated lasers and so on. In many of these demonstrated devices, silicon micrometer-sized optical resonators (optical microresonators) in microring or microdisk form are adopted as the constituent functional elements due to their sharp resonances for wavelength selectivity, compact size, accessibility by integrated optical waveguides and tunability by way of thermo-optical (TO), electro-optical (EO), all-optical and opto-mechanical mechanisms.
Among available silicon refractive index tuning mechanisms, TO and EO effects are the two main mechanisms that are generally employed to dynamically control the silicon microresonator resonance wavelengths, and thus the operation wavelength range of the device. Silicon TO effect features the key merit of enabling a relatively large change in the refractive index n upon a change in temperature T (Δn/ΔT=1.86×10−4 K−1). However, the relatively large thermal conductivity of silicon also makes it difficult to localize the temperature change, and a combination of air-trenches and silicon undercuts surrounding the photonic device are often needed in order to localize the heat for TO tuning. Yet, the speed of thermal diffusion in silicon limits the response time to the μs-range and thus a relatively low modulation speed. Hence, silicon EO effect, which is dominated by the fast free-carrier plasma dispersion effect implemented by accumulating or injecting or depleting free-carrier densities in the optical field region, has become the preferred silicon refractive index tuning mechanism for high-speed optical switches, modulators and other silicon photonic devices that demand ns to sub-ns response times.